The present Invention relates, in general, to novel acoustic sensors and, more particularly, to novel acoustic sensors which include transducers based upon the phenomenon of the microbending of optical fibers.
At the present time, commonly used physical sensors are mainly based upon piezoelectric or magnetostrictive devices. These sensors are all electrically wired systems and, as such, are subject to electromagnetic interference and are difficult to secure.
With the rapid advancement of fiber optic technology (fibers, lasers, detectors, etc.) numerous optical fiber sensors have been developed. For example, phase modulation in single-mode fibers has been utilized as a transduction mechanism for fiber optic sensors such as acoustic, gyro. magnetic, acceleration, and temperature sensors, to name but a few. These sensors are somewhat complex and they utilize single mode fiber technology, some areas of which have yet to be perfected.
In addition to phase modulation, intensity modulation can also be utilized for fiber optic sensing. Intensity modulated sensors include evanescent wave sensors, coupled waveguide sensors, moving fiber optic sensors, Schlieren or grading sensors, polarization sensors, and Fresnel reflection sensors. Most of the known intensity modulated sensors are of a fiber termination type wherein the sensing fiber is terminated in the sensing area with the light leaving the fiber and subsequently being coupled into a second fiber. Such systems have problems efficiently coupling the light into the second fiber, especially in hostile or remote sensing environments.
A very promising transduction mechanism for fiber optic sensors is intensity modulation produced by mode coupling due to microbending along the axis of a multimode fiber. Such sensors are based on intensity modulation of the light power in the core or clad modes produced by a periodic axial deformation of the fiber which introduces strong excess losses due to the coupling of core (guided) modes to radiated modes. Such sensors have been described in the literature by Fields, Asawa, Smith, and Morrison, "Fiber-Optic Hydrophone" Advances in Ceramics, Vol. 2, ED. by B. Bendow and S. Mitra, Publ. Amer. Cer. Soc. 1981, Proc. 82nd Annual Meeting, Amer. Cer. Soc., Chicago, April 1980, p. 529-538; and Lagakos, Macedo, Litovitz, Mohr, and Meister, "Fiber Optic Dispacement Sensor", IBID. p. 539-544. These articles can supply the reader with additional background information regarding microbend sensors and as such are specifically incorporated herein by reference.
In a typical prior art microbending sensor, periodic deformation along the fiber axis can be introduced by means of a pair of corregated plates, called the deformer, which are located on either side of a multimode fiber and which apply pressure thereto. Generally the corregations of the deformer plates would interleave if the optical fiber were removed from between them.
FIG. 1 schematically illustrates such a prior art microbending sensor arrangement and illustrates the effect on light conducted by a fiber due to force from a pressure field applied to a diaphragm and transferred to act on the fiber by means of the deformer. Any relative displacement of the corregated plates of the deformer introduced by an external field, such as an acoustic field, will cause a periodic deformation of the fiber which will result in mode coupling. This mode coupling redistributes the light among core modes and couples some light from the core modes to clad modes. Thus, by monitoring the light power in certain modes, an external force may be detected. These monitored modes may be core modes or clad modes.
FIG. 2 illustrates a prior art acoustic pressure sensor including an intensity-modulated microbend fiber optic sensing element. In this sensor, light from a source, such as a laser or a light emitting diode, is coupled into a multimode fiber and propagates through an acoustically sensitive microbend intensity modulator which surrounds the fiber. FIG. 2(a) illustrates various light ray patterns propagating within a portion of the multimode fiber passing through the microbend intensity modulator.
The characteristics of a microbend sensor, such as, sensitivity, minimum detectable pressure, and resonant frequency, depend strongly upon the acoustical and mechanical design of the sensor and, in particular, depend on the characteristics of the acoustic coupler and the deformer. The acoustic coupler is usually a piston or a diaphragm which multiplies and transfers the pressure of an applied acoustic wave to the sensing element. The design of the deformer which bends the sensing element can vary widely, being periodic or non-periodic, spatically short or extended, in accordance with the desired sensor performance. For compactness and versatility the deformer can even be an integral part of the fiber coating.
After leaving the modulator, the fiber passes through a mode selector which selects the group of modes to be detected by a photodetector such as a pin diode or an avalanche diode. The selected modes for detection may be all the core modes, all the clad modes, either high or low order core modes, or any combination of these groups of modes. Thus, by monitoring the light power in certain modes, an external force can be detected.
The known prior art acoustic sensors, as schematically illustrated in FIG. 2 all suffer from numerous undesirable characteristics. In particular, the mechanical design tends to be rather complex, the alignment of the deformer is critical, the sensor bandwidth is limited, and the sensor performance is often degraded due to acceleration effects within the deformer.